Humidity and wooden pendulums

The tempo of a pendulum depends on its length. Shorter is faster.

All my clocks have wooden pendulums. It's the traditional material and it has the advantage that, unlike steel pendulums, they do not expand much with heat. However wooden pendulums are very sensitive to changes in humidity and they swell and slow down when it is wet. This page describes an attempt to solve this problem and compensate for humidity.

4 June My experimental pendulum →   is a plain pine-wood lath (5 x 27mm) and about a metre long. The pendulum weight is a steel tube 34 x 27 x 250 mm. 0.66kg. The tempo is regulated with an M3 (3mm) nut on a long threaded rod that positions the weight so the clock does one tick a second.
For this experiment I have added a block of wood between the weight and the regulation nut. In this block the grain of the wood is arranged so that the direction of maximum expansion - along the direction of the rings of the tree - runs more or less vertically.
The idea is that the wood block pushes the weight upwards when it is wet, hopefully the same amount as the pendulum lath expands downwards, along the grain. The idea is so simple that I can't imagine that it is original.

Above: Under the pendulum weight is my block of expanding pieces of wood. It is 10cm long. I expect that 10cm will over-compensate for humidity. If so, I will shorten the block of wood until the compensation is correct.

5 June I adjusted the pendulum so that it is running about the same as a nearby uncompensated clock of similar construction.→

6 June Now both clocks are adjusted according to the DCF77 time signal from Frankfurt.
For fine adjustments like this the method is to add or remove small weights from a weight tray half way up the pendulum. For example 0.3 grams = 5 seconds a day for this pendulum. This avoids having to stop the clock to turn the regulation nut; the clock would take several hours to settle down again after being stopped and re-started.

14 June During the first week's run the 10cm wood block followed the humidity readings quite nicely and over-compensated, as expected. I cut the block down to 5cm - carefully saving the cut-off piece - and had readjusted the clock to DCF77 again by the morning of the 15th June.

22 JuneThe changes in rate are still related to humidity. not as wild as previously but far from even so I cut the block to half its previous size; it is now 2.5 cm long. I finished regulating the clock to DCF77 by the morning of the 23rd June.

1 July There has been some improvement with smaller variations. Except for the 30th June the kinks in the clock and humidity graphs change in the same direction.

graph
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